1032 



" Undue distention of the cells is prevented by the constant eva- 

 poration which takes place from the leaves, and a larger quantity of 

 fluid is by this means caused to pass through a plant to furnish it 

 with a sufficient supply of those substances which it requires to ob- 

 tain from the soil. This process, being thus accessory to the function 

 of nutrition, is aided by the stomata, and also by the abundant pu- 

 bescence present on most leaves, and especially on young leaf-buds. 

 Although this evaporation, called sometimes transpiration, must aid 

 indirectly the ascent of the sap, yet of itself it is altogether insuffici- 

 ent to cause it to ascend ; the sap ceasing to ascend as soon as the 

 cells lose their power of elaborating their contents, and thereby keep- 

 ing up the physical conditions necessary for the continuance of 

 endosraose." — p. 10. 



The author here records experiments tried on portions of Anthris- 

 cus vulgaris and Lapsana communis, some of which were placed in 

 solutions of gum arable of different degrees of strength, and others in 

 water: the results obtained were, in the stronger solution the plants 

 ceased to absorb several days sooner than in the weaker, and in this 

 several days sooner than in pure water. 



"25. A solution of sugar acts differently to that of gum. If trans- 

 parent vegetables, when placed in a solution of sugar, be examined 

 by the microscope, the effect is seen to be very remarkable. No 

 plant which I have seen exhibits this effect better than the Nitella. 

 Almost the instant a piece of this plant in which the cyclosis is ac- 

 tive is brought into contact with the solution, its internal membrane 

 becomes partially and suddenly torn from the enclosing cell-wall, its 

 contents escape, and the cyclosis ceases. 



" 26. The experiments first described show that the ascending sap 

 is attracted and not propelled ; and therefore the explanation of the 

 cause of the ascent of the sap given by Dutrochet, as the consequence 

 of the passage of fluid from the earth into the roots by endosmose, 

 must, in these instances, fail. Nor is the explanation advanced by 

 some physiologists, by whom it is supposed that the crude sap as- 

 cends from cell to cell by endosmose, assuming that the contents of 

 the cell above are always more dense than those of the cell below, 

 less at variance with the facts shown by these experiments. As in 

 the stems whose vitality had been destroyed by the bichloride of 

 mercury, along which the sap, or rather the fluid which furnishes the 

 sap, has been shown to ascend with perfect facility, there could not 

 jiossibly exist that di(fei*ence in the density of the contents of the 

 cells at different parts of the stem necessary to ensure the continued 



